JP7653448B2 - Method for producing polarizing film and polarizing film - Google Patents

Description

The present invention relates to a film for producing an optical film, a method for producing an optical film, and an optical film.

Polarizing plates, which have the function of transmitting and blocking light, are basic components of liquid crystal displays (LCDs) along with liquid crystals, which change the polarization state of light. Many polarizing plates have a structure in which a protective film, such as a triacetate cellulose (TAC) film, is laminated to the surface of a polarizing film. The mainstream polarizing film is one in which a dichroic dye, such as an iodine-based dye (I ₃ ^- , I ₅ ^- , etc.) or a dichroic organic dye, is adsorbed onto a matrix (stretched film) made by uniaxially stretching a polyvinyl alcohol film (hereinafter, "polyvinyl alcohol" may be abbreviated as "PVA").

LCDs are used in a wide range of applications, including small devices such as calculators and watches, smartphones, laptops, LCD monitors, LCD color projectors, LCD televisions, in-car navigation systems, mobile phones, and measuring instruments used both indoors and outdoors. In response to the recent increase in performance of LCDs, there is a demand for improved performance in the polarizing plates, which are a component of LCDs. Specifically, there is a demand for polarizing plates that have better optical performance and excellent dimensional stability even at high temperatures. Therefore, there is a demand for polarizing films used in polarizing plates that have better optical performance (polarizing performance) and low shrinkage stress at high temperatures.

However, it is not easy to reduce the shrinkage stress at high temperatures while improving the optical performance (polarization performance) of polarizing films. This is because there is generally a trade-off between the polarization performance and shrinkage stress of polarizing films. In other words, if you try to increase the polarization performance of a polarizing film, the shrinkage stress increases, and if you try to decrease the shrinkage stress, the polarization performance decreases.

Patent Document 1 discloses a polarizing film with improved optical performance and moist heat resistance, which uses PVA having a degree of polymerization of at least 2500. In this patent document 1, a film is formed using a PVA film-forming solution in which PVA with a high degree of polymerization, such as 4980, is dissolved in a solvent mainly composed of dimethyl sulfoxide.

Japanese Unexamined Patent Publication No. 1-105204

In the industrial production of PVA films, it is common to use a PVA aqueous solution in which water is used as a solvent as a film-forming solution, taking into consideration environmental aspects, economics, etc. However, when a PVA with a high degree of polymerization is used as in Patent Document 1, the viscosity of the PVA aqueous solution increases, resulting in poor film-forming properties of the PVA film, which is undesirable for industrial production. Therefore, a method for improving the optical performance of an optical film by a method other than a method for increasing the polymerization degree of PVA is desired. In addition, when the production conditions (stretching conditions) of the optical film are the same, an optical film obtained from a PVA film having a PVA with a high degree of polymerization has a higher shrinkage stress than a PVA film having a PVA with a low degree of polymerization. Therefore, when a PVA film having a PVA with a high degree of polymerization is used, in order to obtain an optical film with excellent optical performance and small shrinkage stress, it is necessary to increase the stretching temperature when producing the optical film, which is undesirable for industrial production. Therefore, even when a PVA film having a PVA with a high degree of polymerization is used, a method for obtaining an optical film with excellent optical performance without increasing the stretching temperature when producing the optical film is desired.

The present inventors have found that the use of a PVA film containing PVA having a silicon-containing group can significantly improve the optical performance of the resulting optical film while suppressing an increase in the viscosity of the PVA aqueous solution. However, even when a PVA film containing PVA having a silicon-containing group is used, it is still necessary to increase the stretching temperature when producing the optical film.

The present invention has been made based on the above circumstances, and its object is to provide a film for producing an optical film that, when producing an optical film with a similar shrinkage stress, can suppress the stretching temperature from being increased and produce an optical film with excellent optical performance, a method for producing an optical film using such a film for producing an optical film, and an optical film.

The above objectives are:
[1] A film for producing an optical film, comprising a PVA having a silicon-containing group and an ethylene unit, the silicon-containing group being a silanol group or a group that can be converted into a silanol group in the presence of water, the content of the silicon-containing group relative to all structural units in the PVA being 0.01 mol % or more and 1.0 mol % or less, and the content of the ethylene unit being 0.5 mol % or more and 10 mol % or less;
[2] The film for producing an optical film according to [1], wherein the PVA has a viscosity average degree of polymerization of 1,000 or more and 6,000 or less, and a degree of saponification of 98.7 mol% or more;
[3] The film for producing an optical film according to [2], wherein the product of the viscosity average degree of polymerization and the content of the silicon-containing group is 100 mol % or more and 2,000 mol % or less;
[4] The film for producing an optical film according to any one of [1] to [3], having an average thickness of 1 μm or more and 75 μm or less;
[5] The film for producing an optical film according to any one of [1] to [4], having a swelling degree of 140% or more and 400% or less;
[6] The film for producing an optical film according to any one of [1] to [5], wherein the optical film is a polarizing film;
[7] The film for producing an optical film according to any one of [1] to [6], which is a non-stretched film;
[8] A method for producing an optical film, comprising a step of uniaxially stretching the film for optical film production according to any one of [1] to [7];
[9] The method for producing an optical film according to [8], wherein the optical film is a polarizing film;
[10] An optical film comprising a PVA having a silicon-containing group and an ethylene unit, the silicon-containing group being a silanol group or a group that can be converted into a silanol group in the presence of water, the content of the silicon-containing group relative to all structural units in the PVA being 0.01 mol% or more and 1.0 mol% or less, and the content of the ethylene unit being 0.5 mol% or more and 10 mol% or less;
[11] The optical film according to [10], having a shrinkage stress of 100 N/m2 or ^less ;
[12] The optical film of [10] or [11], which is a polarizing film; and [13] the optical film of any one of [10] to [12], which is a single-layer film;
This is achieved by providing either

According to the present invention, when producing optical films with the same degree of shrinkage stress, it is possible to provide a film for producing an optical film that can suppress an increase in the stretching temperature and obtain an optical film with excellent optical performance, a method for producing an optical film using such a film for producing an optical film, and an optical film. In other words, according to the film for producing an optical film and the method for producing an optical film of the present invention, it is possible to suppress an increase in shrinkage stress and an increase in the stretching temperature while improving the optical performance of the obtained optical film.

FIG. 1 is a graph in which the stretching temperature at which the shrinkage stress of each of the polarizing films obtained in Examples 1 to ³ and Comparative Examples 1 to 5 becomes 75 N/mm2 is plotted against the dichroic ratio at that time. FIG. 2 is a graph in which the stretching temperature at which the shrinkage stress of each of the polarizing films obtained in Example 3 and Comparative Examples 1, 4, and ⁵ becomes 75 N/mm2 is plotted against the dichroic ratio at that time.

<Film for optical film manufacturing>
The film for producing an optical film of the present invention contains polyvinyl alcohol having a silicon-containing group and an ethylene unit (hereinafter, sometimes referred to as "modified PVA").

(Modified PVA)
Modified PVA is a polymer having vinyl alcohol units (-CH ₂ -CH(OH)-) as structural units, and also has a silicon-containing group and an ethylene unit. The modified PVA may further contain vinyl ester units such as vinyl acetate units and other structural units.

The lower limit of the viscosity average polymerization degree of the modified PVA is preferably 1,000, more preferably 2,000, and even more preferably 2,500. When the viscosity average polymerization degree of the modified PVA is equal to or greater than the lower limit, the film for optical film production of the present invention has excellent stretching processability, and an optical film having higher optical performance and good moist heat resistance can be obtained. On the other hand, the upper limit of the viscosity average polymerization degree is preferably 6,000, more preferably 5,000, and even more preferably 4,000. When the viscosity average polymerization degree of the modified PVA is equal to or less than the upper limit, good water solubility is exhibited and the increase in viscosity of the aqueous solution is suppressed. In addition, when the viscosity average polymerization degree of the modified PVA is equal to or less than the upper limit, the stretching temperature is further suppressed from increasing. Therefore, when the viscosity average polymerization degree of the modified PVA is equal to or less than the upper limit, good film formability is exhibited, and the productivity of the film for optical film production of the present invention can be increased. Furthermore, when the viscosity average polymerization degree of the modified PVA is equal to or less than the upper limit, an optical film in which an increase in shrinkage stress is suppressed can be easily obtained from the film for producing an optical film of the present invention.

The viscosity average degree of polymerization means the average degree of polymerization measured in accordance with the description of JIS K6726-1994. That is, in this specification, the viscosity average degree of polymerization is calculated from the intrinsic viscosity [η] (unit: deciliter/g) measured in water at 30° C. after resaponifying the remaining vinyl ester groups of PVA and purifying it, according to the following formula:
Viscosity average degree of polymerization Po=([η]×10 ⁴ /8.29) ^(1/0.62)

The lower limit of the saponification degree of the modified PVA is preferably 98.7 mol%, more preferably 99.0 mol%, even more preferably 99.5 mol%, even more preferably 99.8 mol%, and particularly preferably 99.9 mol%. By having a saponification degree equal to or greater than the lower limit, an optical film having superior optical performance and moist heat resistance can be obtained. On the other hand, there is no particular limit to the upper limit of the saponification degree, but from the viewpoint of productivity of the modified PVA, 99.99 mol% or less is preferable.

The degree of saponification of PVA refers to the ratio (mol%) of the number of moles of vinyl alcohol units to the total number of moles of structural units (typically vinyl ester units) that can be converted to vinyl alcohol units by saponification and vinyl alcohol units that the PVA has. The degree of saponification of PVA can be measured in accordance with the description of JIS K6726-1994.

The modified PVA has a silicon-containing group. This silicon-containing group is a silanol group or a group that can be converted to a silanol group in the presence of water. A silanol group is a group that has a silicon atom and at least one hydroxyl group bonded to the silicon atom. The number of hydroxyl groups in the silanol group is usually 1 to 3, and preferably 3. The hydroxyl group in the silanol group may be present in the form of a salt (e.g., -ONa, -OK, etc.).

The group that can be converted to a silanol group in the presence of water means a group that can be converted to a silanol group when PVA is heated in water under conditions of a reaction time of 2 hours and a reaction temperature of 150°C. This conversion to a silanol group may occur by hydrolysis. Examples of groups that can be converted to a silanol group in the presence of water include groups in which at least one alkoxy group or acyloxy group is bonded to a silicon atom, and specific examples include trimethoxysilyl groups, triethoxysilyl groups, triisopropoxysilyl groups, dimethoxymethylsilyl groups, diethoxymethylsilyl groups, methoxydimethylsilyl groups, ethoxydimethylsilyl groups, and triacetoxysilyl groups.

Examples of silicon-containing groups, i.e., silanol groups or groups that can be converted to silanol groups in the presence of water, include groups represented by any of the following formulas (1) to (3). Among these, the group represented by the following formula (1) is preferred.

In formulas (1) to (3), each R ¹ is independently a hydrogen atom, a substituted or unsubstituted hydrocarbon group having 1 to 20 carbon atoms, or a substituted or unsubstituted acyl group having 1 to 20 carbon atoms. Each R ² is independently a substituted or unsubstituted hydrocarbon group having 1 to 20 carbon atoms.

Examples of the hydrocarbon group having 1 to 20 carbon atoms represented by ^R1 and ^R2 include aliphatic hydrocarbon groups, alicyclic hydrocarbon groups (cyclohexyl group, etc.), and aromatic hydrocarbon groups (phenyl group, etc.), with aliphatic hydrocarbon groups being preferred. Examples of the aliphatic hydrocarbon group include alkyl groups such as methyl, ethyl, and propyl groups, alkenyl groups such as vinyl groups, and alkynyl groups such as ethynyl groups, with alkyl groups being preferred. The number of carbon atoms in the hydrocarbon group represented by ^R1 and ^R2 is preferably 1 to 6, and more preferably 1 to 3. At least a portion of the hydrogen atoms in the hydrocarbon group represented by ^R1 and ^R2 may be substituted with a halogen atom, a carboxy group, an alkoxy group (methoxy group, ethoxy group, etc.), or the like.

Examples of the acyl group having 1 to 20 carbon atoms represented by ^R1 include a hydrogen atom or a group in which a carbonyl group (-CO-) is bonded to a hydrocarbon group having 1 to 19 carbon atoms. The hydrocarbon group having 1 to 19 carbon atoms is preferably an aliphatic hydrocarbon group, and more preferably an alkyl group. Specific examples of the acyl group include a formyl group, an acetyl group, a propionyl group, and a benzoyl group. The number of carbon atoms in the acyl group represented by ^R1 is preferably 1 to 6, and more preferably 1 to 3. At least a portion of the hydrogen atoms in the acyl group represented by ^R1 may be substituted with a halogen atom, a carboxy group, an alkoxy group (such as a methoxy group), or the like.

In the group represented by any one of the formulas (1) to (3), when at least one R ¹ is a hydrogen atom, the group is a silanol group. In addition, in the group represented by any one of the formulas (1) to (3), when all R ¹ are not hydrogen atoms, the group is a group that can be converted to a silanol group in the presence of water. R ¹ is preferably a hydrogen atom or a substituted or unsubstituted hydrocarbon group having 1 to 20 carbon atoms.

From the viewpoint of the optical performance of the resulting optical film, it is preferable that the silicon-containing group is directly bonded to a carbon atom in the polymer main chain via a silicon-carbon bond.

The content of silicon-containing groups relative to all structural units in the modified PVA is 0.01 mol% or more and 1.0 mol% or less. In this way, by using a modified PVA having a predetermined amount of silicon-containing groups, the optical performance of the obtained optical film is improved. The lower limit of the content of silicon-containing groups in the modified PVA is 0.01 mol%, preferably 0.05 mol%, more preferably 0.1 mol%, and even more preferably 0.15 mol%. By making the content of silicon-containing groups equal to or greater than the lower limit, the optical performance of the obtained optical film can be sufficiently improved. On the other hand, the upper limit of the content of silicon-containing groups relative to all structural units in the modified PVA is 1.0 mol%, preferably 0.8 mol%, more preferably 0.6 mol%, and even more preferably 0.4 mol%. By making the content of silicon-containing groups equal to or less than the upper limit, the water solubility of the modified PVA and the viscosity stability of the aqueous solution are improved, and film productivity (film formability) can be improved. Furthermore, by controlling the content of the silicon-containing group to be equal to or less than the upper limit, it is also possible to prevent the stretching temperature from becoming too high.

In the modified PVA, the content (mol%) of the silicon-containing group can be determined, for example, from proton NMR of the vinyl ester polymer before saponification. In measuring the proton NMR of the vinyl ester polymer before saponification, the vinyl ester polymer is purified by reprecipitation with hexane-acetone to completely remove unreacted monomers from the polymer, then dried at 90° C. under reduced pressure for 2 days, and then dissolved in CDCl ₃ solvent for analysis.

The lower limit of the product of the viscosity average polymerization degree of the modified PVA and the content of silicon-containing groups is preferably 100 mol%, more preferably 200 mol%, even more preferably 300 mol%, and even more preferably 400 mol%. When the product is equal to or greater than the lower limit, the optical performance of the resulting optical film is superior. On the other hand, the upper limit of the product is preferably 2,000 mol%, more preferably 1,500 mol%, even more preferably 1,200 mol%, and particularly preferably 800 mol%. When the product is equal to or less than the upper limit, the water solubility of the modified PVA is increased and the stretching temperature is further suppressed from increasing, thereby making it possible to further increase the productivity of the film for producing the optical film.

The modified PVA preferably has a structural unit having a silicon-containing group. An example of the structural unit having a silicon-containing group is a structural unit represented by the following formula (4).

In formula (4), ^R3 is a hydrogen atom or a methyl group, ^R4 is a single bond or a divalent linking group, and ^R5 is a silicon-containing group.

^R3 is preferably a hydrogen atom.

Examples of the divalent linking group represented by ^R4 include groups represented by -( _CH2 ) _n- (n is an integer of 1 to 5) or ^-CONR6 - ^R7- ( ^R6 is a hydrogen atom or an alkyl group having 1 to 5 carbon atoms, and ^R7 is the group represented by -( _CH2 ) _n- or a divalent hydrocarbon group containing at least one of an oxygen atom and a nitrogen atom).

Examples of divalent hydrocarbon groups _containing at least _one of an oxygen atom _and a nitrogen atom include _{-CH2CH2NHCH2CH2CH2-} , -CH2CH2NHCH2CH2- _, -CH2CH2NHCH2CH2- _, -CH2CH2NHCH2-, -CH2CH2N ₍ CH3 _{) CH2CH2- ,} -CH2CH2N ₍ CH3) _{CH2-, -CH2CH2OCH2CH2CH2-} , _{-CH2CH2OCH2CH2- , -CH2CH2OCH2CH2- ,} etc. _{The number} of carbon atoms _{in the divalent} hydrocarbon group containing at least one _{of an oxygen atom} and _a nitrogen atom can be _, for example _, 2 or _{more and 6 or} less.

^R4 is preferably a single bond.

Specific examples of the silicon-containing group represented by ^R5 are as described above, and include the groups represented by any one of the formulas (1) to (3), with the group represented by the formula (1) being preferred.

The number of silicon-containing groups contained in the structural unit having a silicon-containing group is not particularly limited, but may be 1. The range of the content of the structural unit having a silicon-containing group relative to all structural units in the modified PVA may be the range of the content of the silicon-containing group described above. In addition, the range of the product of the viscosity average degree of polymerization of the modified PVA and the content of the structural unit having a silicon-containing group may be the range of the product of the viscosity average degree of polymerization and the content of the silicon-containing group described above.

The modified PVA further has an ethylene unit (-CH ₂ -CH ₂ -). The content of the ethylene unit relative to all structural units in the modified PVA is 0.5 mol% or more and 10 mol% or less. In this way, by using a modified PVA having a predetermined amount of ethylene units, an optical film having excellent optical performance can be obtained while suppressing the increase in the stretching temperature. The lower limit of the content of the ethylene unit is preferably 1.0 mol%, more preferably 2.0 mol%. By making the content of the ethylene unit equal to or more than the lower limit, the increase in the stretching temperature can be further suppressed. On the other hand, the upper limit of the content of the ethylene unit is preferably 6 mol%, more preferably 5 mol, and even more preferably 4.5 mol%. By making the content of the ethylene unit equal to or less than the upper limit, the water solubility of the modified PVA can be increased.

The ethylene unit content in the modified PVA can be determined, for example, using proton NMR. For example, in the same manner as the silicon-containing group content described above, the ethylene unit content of the modified PVA can be determined by measuring the modified vinyl ester polymer, which is the precursor of the modified PVA, using proton NMR.

The modified PVA may have other structural units other than vinyl alcohol units, vinyl ester units, structural units having silicon-containing groups, and ethylene units. However, the content of the other structural units relative to the total structural units of the modified PVA is preferably 15 mol% or less, more preferably 5 mol% or less, even more preferably 1 mol% or less, and even more preferably 0.1 mol% or less. When the modified PVA is substantially composed of vinyl alcohol units, vinyl ester units, structural unit groups having silicon content, and structural units having ethylene units, the effects of the present invention may be more fully achieved.

The film for producing optical films may contain one type of modified PVA alone, or may contain two or more types of modified PVA that differ from each other in degree of polymerization, degree of saponification, content of silicon-containing groups, content of ethylene units, etc.

The lower limit of the content of modified PVA in the film for optical film production is not particularly limited, but is preferably 50% by mass, more preferably 80% by mass, and even more preferably 85% by mass. By making the content of modified PVA equal to or greater than the lower limit, the effect of the present invention can be further enhanced. On the other hand, the upper limit of this content is not particularly limited, and may be 100% by mass, preferably 99% by mass, and more preferably 95% by mass. The content (mass%) of each component in the film for optical film production is based on the dry state, i.e., the total content of all components other than water.

(Method for producing modified PVA)
The method for producing the modified PVA is not particularly limited. For example, the modified PVA can be produced by copolymerizing a vinyl ester monomer, a monomer having a silicon-containing group, and ethylene, and saponifying the resulting vinyl ester polymer.

Examples of vinyl ester monomers include vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl caprate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl pivalate, and vinyl versatate. Of these, vinyl acetate is preferred.

Monomers having silicon-containing groups include vinyltrimethoxysilane, vinylmethyldimethoxysilane, vinyldimethylmethoxysilane, vinyltriethoxysilane, vinylmethyldiethoxysilane, vinyldimethylethoxysilane, allyltrimethoxysilane, allylmethyldimethoxysilane, allyldimethylmethoxysilane, allyltriethoxysilane, allylmethyldiethoxysilane, allyldimethylethoxysilane, vinyltris(β-methoxyethoxy)silane, vinylisobutyldimethoxysilane, vinylethyldimethoxysilane, vinylmethoxydibutoxysilane, vinyldimethoxybutoxysilane, and vinyltributoxysilane. , vinyl methoxydihexyloxysilane, vinyl dimethoxyhexyloxysilane, vinyl trihexyloxysilane, vinyl methoxydioctyloxysilane, vinyl dimethoxyoctyloxysilane, vinyl trioctyloxysilane, vinyl methoxydilauryloxysilane, vinyl dimethoxylauryloxysilane, vinyl methoxydioleyloxysilane, vinyl dimethoxyoleyloxysilane, 3-(meth)acrylamide-propyl trimethoxysilane, 3-(meth)acrylamide-propyl triethoxysilane, 3-(meth)acrylamide-propyl tri(β-methoxyethoxy)silane, 2-(meth)acrylamide-ethyl trimethoxysilane Silane, 1-(meth)acrylamide-methyltrimethoxysilane, 2-(meth)acrylamide-2-methylpropyltrimethoxysilane, 2-(meth)acrylamide-isopropyltrimethoxysilane, N-(2-(meth)acrylamide-ethyl)-aminopropyltrimethoxysilane, (3-(meth)acrylamide-propyl)-oxypropyltrimethoxysilane, 3-(meth)acrylamide-propyltriacetoxysilane, 2-(meth)acrylamide-ethyltriacetoxysilane, 4-(meth)acrylamide-butyltriacetoxysilane, 3-(meth)acrylamide-propyltripropionyloxysilane , 2-(meth)acrylamide-2-methylpropyltriacetoxysilane, N-(2-(meth)acrylamide-ethyl)-aminopropyltriacetoxysilane, 3-(meth)acrylamide-propylisobutyldimethoxysilane, 2-(meth)acrylamide-ethyldimethylmethoxysilane, 3-(meth)acrylamide-propylmethyldiacetoxysilane, 2-(meth)acrylamide-2-methylpropylhydrogendimethoxysilane, 3-(N-methyl-(meth)acrylamide)-propyltrimethoxysilane, 2-(N-ethyl-(meth)acrylamide)-ethyltriacetoxysilane, and the like.

The method for copolymerizing the vinyl ester monomer, the monomer having a silicon-containing group, and ethylene is not particularly limited, and includes known methods such as bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization. Among these methods, bulk polymerization carried out without a solvent and solution polymerization carried out using a solvent such as alcohol are preferred. Solvents used as solvents during solution polymerization include esters such as methyl acetate and ethyl acetate; aromatic hydrocarbons such as benzene and toluene; and lower alcohols such as methanol and ethanol.

The initiator used in the copolymerization reaction is appropriately selected from conventionally known azo initiators, peroxide initiators, redox initiators, etc. Examples of azo initiators include 2,2'-azobisisobutyronitrile, 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), etc. Examples of the peroxide initiator include percarbonate compounds such as di-normal propyl peroxydicarbonate, diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, and diethoxyethyl peroxydicarbonate; perester compounds such as t-butyl peroxyneodecanate, α-cumyl peroxyneodecanate, and t-butyl peroxyneodecanate; acetylcyclohexylsulfonyl peroxide, diisobutyryl peroxide; and 2,4,4-trimethylpentyl-2-peroxyphenoxyacetate. Furthermore, the above-listed peroxide initiators can be combined with potassium persulfate, ammonium persulfate, hydrogen peroxide, and the like to form an initiator. Examples of the redox initiator include combinations of the above-listed peroxides with reducing agents such as sodium hydrogen sulfite, sodium hydrogen carbonate, tartaric acid, L-ascorbic acid, and Rongalite.

There are no particular restrictions on the polymerization temperature when carrying out the copolymerization reaction, but a temperature of 0°C or higher and 180°C or lower is preferred, a temperature of 20°C or higher and 160°C or lower is more preferred, and a temperature of 30°C or higher and 150°C or lower is even more preferred.

A preferred method for producing a vinyl ester polymer is copolymerization of ethylene under pressure in the presence of a vinyl ester monomer and a monomer having a silicon-containing group. The ethylene pressure in the polymerization reactor is not particularly limited, but is preferably 0.01 to 2.0 MPa, more preferably 0.05 to 1.0 MPa, and in some cases even more preferably 0.1 to 0.65 MPa. The polymerization rate of the vinyl ester monomer at the outlet of the polymerization reactor is not particularly limited, but is preferably 5 to 90%, and in some cases even more preferably 15 to 85%.

When copolymerizing a vinyl ester monomer, a monomer having a silicon-containing group, and ethylene, other copolymerizable monomers can be copolymerized as necessary, as long as the effects of the present invention are not impaired. Examples of such other monomers include olefins having 3 to 30 carbon atoms, such as propylene, 1-butene, and isobutene; acrylic acid or its salts; acrylic acid esters such as methyl acrylate, ethyl acrylate, n-propyl acrylate, i-propyl acrylate, n-butyl acrylate, i-butyl acrylate, t-butyl acrylate, 2-ethylhexyl acrylate, dodecyl acrylate, and octadecyl acrylate; methacrylic acid or its salts; methyl methacrylate, ethyl methacrylate, and n-propyl methacrylate. methacrylic acid esters such as i-propyl methacrylate, n-butyl methacrylate, i-butyl methacrylate, t-butyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate, and octadecyl methacrylate; acrylamide, N-methylacrylamide, N-ethylacrylamide, N,N-dimethylacrylamide, diacetone acrylamide, acrylamidopropanesulfonic acid or a salt thereof, acrylamidopropyldimethylamine or a salt thereof, N-methylolacrylamide or a salt thereof, methacrylamide derivatives such as methacrylamide, N-methyl methacrylamide, N-ethyl methacrylamide, methacrylamide propanesulfonic acid or a salt thereof, methacrylamide propyl dimethylamine or a salt thereof, N-methylol methacrylamide or a derivative thereof; N-vinyl amides such as N-vinyl formamide, N-vinyl acetamide, and N-vinyl pyrrolidone; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, n-butyl vinyl ether, i-butyl vinyl ether, t-butyl vinyl ether, dodecyl vinyl ether, and stearyl vinyl ether; vinyl cyanides such as acrylonitrile and methacrylonitrile; vinyl halides such as vinyl chloride, vinylidene chloride, vinyl fluoride, and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; maleic acid or a salt, ester, or acid anhydride thereof; itaconic acid or a salt, ester, or acid anhydride thereof; and isopropenyl acetate. The vinyl ester polymer may have structural units derived from one or more of the other monomers mentioned above.

The proportion of structural units derived from the other monomers (vinyl ester monomers, monomers having a silicon-containing group, and monomers other than ethylene) in the vinyl ester polymer is not necessarily limited as long as it does not interfere with the effects of the present invention, but may be preferably 15 mol % or less, more preferably 5 mol % or less, even more preferably 1 mol % or less, and even more preferably 0.1 mol % or less, based on the number of moles of all structural units constituting the vinyl ester polymer.

The vinyl ester polymer is then saponified in a solvent according to a known method to produce a modified PVA. The solvent used in the saponification reaction is preferably an alcohol. Examples of alcohol include lower alcohols such as methanol and ethanol, with methanol being particularly preferred. The solvent used in the saponification reaction may further contain, in addition to alcohol, acetone, esters such as methyl acetate and ethyl acetate, and organic solvents such as toluene. Examples of catalysts used in the saponification reaction include alkali metal hydroxides such as potassium hydroxide and sodium hydroxide, alkali catalysts such as sodium methylate, and acid catalysts such as mineral acids. The temperature of the saponification reaction can be, for example, 20°C or higher and 60°C or lower. If a gel-like product precipitates as the saponification reaction proceeds, the product is crushed at that point, washed, and then dried to obtain a modified PVA.

(Plasticizer)
The film for optical film production of the present invention preferably contains a plasticizer. When the film for optical film production contains a plasticizer, it is possible to improve the stretchability. The plasticizer is preferably a polyhydric alcohol. Examples of the polyhydric alcohol include ethylene glycol, glycerin, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, and trimethylolpropane. Among these, glycerin is preferred from the viewpoint of the effect of improving the stretchability. One or more kinds of plasticizers can be used.

The lower limit of the content of the plasticizer in the film for optical film production of the present invention is preferably 1 part by mass, more preferably 3 parts by mass, and even more preferably 5 parts by mass, per 100 parts by mass of modified PVA. By having the content of the plasticizer be equal to or greater than the lower limit, the stretchability of the film is improved, and the optical performance of the obtained optical film can be further improved. On the other hand, the upper limit of the content of the plasticizer is preferably 20 parts by mass, more preferably 17 parts by mass, and even more preferably 15 parts by mass, per 100 parts by mass of modified PVA. By having the content of the plasticizer be equal to or less than the upper limit, it is possible to prevent the film from becoming too flexible and reducing the handleability.

(Surfactant)
The optical film-producing film preferably contains a surfactant. By forming a film using a film-forming solution containing a surfactant, the film-forming properties are improved, the occurrence of unevenness in the film thickness is suppressed, and the film is easily peeled off from a metal roll or belt used in film formation. When a film for optical film production is produced from a film-forming solution containing a surfactant, the resulting film may contain a surfactant.

The type of surfactant is not particularly limited, but anionic surfactants and nonionic surfactants are preferred from the standpoint of releasability from metal rolls and belts.

Examples of anionic surfactants include carboxylic acid types such as potassium laurate; sulfate ester types such as polyoxyethylene lauryl ether sulfate and octyl sulfate; and sulfonic acid types such as dodecylbenzenesulfonate.

Examples of nonionic surfactants include alkyl ether types such as polyoxyethylene oleyl ether; alkyl phenyl ether types such as polyoxyethylene octylphenyl ether; alkyl ester types such as polyoxyethylene laurate; alkyl amine types such as polyoxyethylene lauryl amino ether; alkyl amide types such as polyoxyethylene lauric acid amide; polypropylene glycol ether types such as polyoxyethylene polyoxypropylene ether; alkanolamide types such as lauric acid diethanolamide and oleic acid diethanolamide; and allyl phenyl ether types such as polyoxyalkylene allyl phenyl ether.

Surfactants can be used alone or in combination of two or more.

When the optical film manufacturing film contains a surfactant, the lower limit of the content is preferably 0.01 parts by mass, more preferably 0.02 parts by mass, and even more preferably 0.05 parts by mass, relative to 100 parts by mass of modified PVA. By making the content of the surfactant equal to or greater than the lower limit, the film-forming property and peelability are further improved. On the other hand, the upper limit of the content is preferably 0.5 parts by mass, more preferably 0.3 parts by mass, and even more preferably 0.1 parts by mass, relative to 100 parts by mass of modified PVA. By making the content of the surfactant equal to or less than the upper limit, it is possible to prevent the surfactant from bleeding out onto the surface of the film, causing blocking and reducing handleability.

(Other additives, etc.)
The film for producing an optical film of the present invention may further contain additives, such as a filler, a processing stabilizer such as a copper compound, a weather resistance stabilizer, a colorant, an ultraviolet absorber, a light stabilizer, an antioxidant, an antistatic agent, a flame retardant, other thermoplastic resins, a lubricant, a fragrance, an antifoaming agent, a deodorant, an extender, a peeling agent, a releasing agent, a reinforcing agent, a crosslinking agent, a mildew inhibitor, a preservative, and a crystallization rate retarder, as necessary.

The film for optical film production of the present invention may contain a PVA other than the modified PVA described above. The total content of all PVAs including modified PVA, plasticizers and surfactants in the film for optical film production of the present invention is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and even more preferably 99% by mass or more. When the film for optical film production of the present invention is substantially composed of PVA, plasticizer and surfactant, the effects of the present invention can be more fully exhibited.

The content of the above-mentioned modified PVA relative to all PVA contained in the film for producing an optical film of the present invention is preferably 50% by mass or more, more preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and even more preferably 99% by mass or more. By mainly using modified PVA as the PVA in the film for producing an optical film of the present invention, the effects of the present invention can be more fully exhibited.

The total content of the modified PVA, plasticizer, and surfactant in the film for optical film production of the present invention is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and in some cases even more preferably 99% by mass or more. When the film for optical film production of the present invention is substantially composed of modified PVA, plasticizer, and surfactant, the effects of the present invention can be more fully exhibited.

(shape, physical properties, etc.)
The film for producing an optical film of the present invention is a so-called original film used as a material for an optical film. However, the film for producing an optical film of the present invention is not limited to a roll-shaped film.

The average thickness of the film for optical film production of the present invention is not particularly limited, but the lower limit is preferably 1 μm, more preferably 5 μm, and even more preferably 10 μm. By having an average thickness equal to or greater than the lower limit, it is possible to suppress breakage during the uniaxial stretching process when producing the optical film. Furthermore, the upper limit of this average thickness is preferably 75 μm, more preferably 60 μm, even more preferably 45 μm, and even more preferably 35 μm. By having an average thickness equal to or less than the upper limit, it is possible to suppress stretching unevenness during the uniaxial stretching process. Note that the "average thickness" refers to the average value of thicknesses measured at any five points (the same applies to the average thickness below).

The film for optical film production of the present invention may be a single layer film consisting of one PVA layer (a layer containing modified PVA) or a multilayer film containing one PVA layer. However, when used for producing a polarizing film, it is preferable to use a single layer film. The lower limit of the average thickness of the PVA layer of the film for optical film production of the present invention is preferably 1 μm, more preferably 5 μm, and even more preferably 10 μm. By having the average thickness be equal to or greater than the lower limit, it is possible to suppress breakage during the uniaxial stretching process when producing an optical film. In addition, the upper limit of this average thickness is preferably 75 μm, more preferably 60 μm, even more preferably 45 μm, and even more preferably 35 μm. By having the average thickness be equal to or less than the upper limit, it is possible to suppress stretching unevenness during the uniaxial stretching process.

The specific composition and preferred composition of the PVA layer in the film for producing optical films can be referenced from the description of the specific composition and preferred composition of the film for producing optical films itself described above.

When the film for optical film production of the present invention is a single layer film, the average thickness is preferably 20 μm or more, more preferably 30 μm or more, in order to ensure handling properties. On the other hand, when the film for optical film production of the present invention is a multilayer film, the average thickness of the PVA layer can be 20 μm or less, or 15 μm or less.

A multilayer film refers to a film having two or more layers. The number of layers of a multilayer film may be five or less, or three or less. An example of a multilayer film is a film for manufacturing an optical film having a laminated structure of a base resin layer and a PVA layer. The average thickness of the base resin layer is, for example, 20 μm or more and 500 μm or less. It is preferable that the base resin layer in the multilayer film can be uniaxially stretched together with the PVA layer. As the resin constituting the base resin layer, polyester, polyolefin, etc. can be used. Among them, amorphous polyester resin is preferable, and amorphous polyester resin obtained by copolymerizing polyethylene terephthalate with a copolymerization component such as isophthalic acid or 1,4-cyclohexanedimethanol is preferably used. An adhesive layer may be provided between the base resin layer and the PVA layer.

The width of the film for optical film production of the present invention is not particularly limited and can be determined depending on the application. For example, the lower limit of the width of the film for optical film production is preferably 3 m. In recent years, the screen size of liquid crystal televisions and liquid crystal monitors has been increasing, so if the width of the film for optical film production is 3 m or more, it is suitable for applications in which these are used as final products. On the other hand, the upper limit of the width of the film for optical film production is preferably 7 m. By setting the width to 7 m or less, when producing optical films using practical equipment, it is possible to efficiently perform uniaxial stretching processing.

From the viewpoint of the productivity and optical performance of the optical film, the swelling degree of the film for optical film production of the present invention is preferably in the range of 140% or more and 400% or less. The lower limit of the swelling degree is more preferably 180%, and even more preferably 190%. The upper limit of the swelling degree is more preferably 220%, and even more preferably 210%. The swelling degree of the film can be adjusted to a smaller value, for example, by strengthening the heat treatment conditions.

Here, the "degree of swelling of the film" refers to a value calculated by the following formula.
Swelling degree (%) = 100 x N/M
In the formula, N represents the mass (g) of a sample taken from the film after immersing the sample in distilled water at 30° C. for 30 minutes and removing the surface water, and M represents the mass (g) of the sample after drying the sample in a dryer at 105° C. for 16 hours.

The film for optical film production of the present invention is usually a film that is not substantially stretched (non-stretched film, unstretched film). The in-plane retardation of the film for optical film production is preferably 100 nm or less, more preferably 50 nm or less. Usually, an optical film can be obtained by subjecting the film for optical film production of the present invention to a stretching process (uniaxial stretching process or biaxial stretching process).

When an optical film having the same shrinkage stress is produced by the film for optical film production of the present invention, the stretching temperature is prevented from being raised, and an optical film having excellent optical performance can be obtained. The optical performance includes light transmittance, polarization property, etc. Examples of optical films that can be produced by the film for optical film production include polarizing films, retardation films, viewing angle improving films, brightness improving films, etc., and polarizing films are preferable.

<Method of manufacturing film for producing optical film>
The method for producing the film for optical film production of the present invention is not particularly limited, but a method that makes the thickness and width of the film more uniform after film production can be preferably adopted. For example, the film can be produced using a film-forming solution in which modified PVA and, if necessary, one or more of a plasticizer, a surfactant, and other additives are dissolved in a liquid medium. When the film-forming solution contains at least one of a plasticizer, a surfactant, and other additives, it is preferable that these components are uniformly mixed.

Examples of liquid media used in the preparation of the film-forming solution include water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene glycol, glycerin, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, trimethylolpropane, ethylenediamine, and diethylenetriamine, and one or more of these can be used. Among these, water is preferred in terms of environmental load and recyclability. In addition, the above-mentioned modified PVA has good water solubility, and the increase in viscosity when it is made into an aqueous solution at a relatively high temperature (e.g., 80°C) is also suppressed. From this point of view, water can be preferably used as the liquid medium.

The volatile fraction of the film-forming solution (the content in the film-forming solution of volatile components such as liquid media that are removed by volatilization or evaporation during film formation) is, for example, preferably 50% by mass or more and 95% by mass or less, more preferably 55% by mass or more and 90% by mass or less, and even more preferably 60% by mass or more and 85% by mass or less. When the volatile fraction of the film-forming solution is 50% by mass or more, the viscosity of the film-forming solution does not become too high, and filtration and degassing during preparation of the film-forming solution are performed smoothly, making it easier to produce a film with fewer foreign matter and defects. On the other hand, when the volatile fraction of the film-forming solution is 95% by mass or less, the concentration of the film-forming solution does not become too low, making it easier to produce a film industrially.

The temperature of the film-forming solution during film formation can be, for example, from 60°C to 150°C, and preferably from 70°C to 130°C. By using modified PVA and forming the film at such a relatively high temperature, the viscosity of the film-forming solution can be kept relatively low, and film-forming properties can be improved.

Examples of methods for producing a film using a film-forming solution include cast film-forming, extrusion film-forming, wet film-forming, and gel film-forming. These film-forming methods may be used alone or in combination of two or more. Among these film-forming methods, cast film-forming and extrusion film-forming are preferred because they can produce a film with uniform thickness and width and good physical properties. The produced film can be dried or heat-treated as necessary.

Specific examples of the manufacturing method of the film for optical film manufacturing of the present invention include the following. Using a T-type slit die, a hopper plate, an I-die, a lip coater die, or the like, the film-forming solution is uniformly discharged or cast onto the peripheral surface of a heated first roll (or belt) located on the most upstream side. The volatile components are evaporated from one side of the film discharged or cast onto the peripheral surface of the first roll (or belt) to dry it. The film is then further dried on the peripheral surface of one or more heated rotating rolls located downstream, or further dried by passing through a hot air dryer. The film is then wound up by a winding device. Drying by a heated roll and drying by a hot air dryer may be performed in an appropriate combination.

When the film for optical film production of the present invention is a multilayer film, the multilayer film can be produced, for example, by applying a film-forming solution onto a base resin film (base resin layer). At this time, in order to improve the adhesion between the PVA layer and the base resin layer, the surface of the base resin film may be modified or an adhesive may be applied to the surface of the base resin film.

<Method of Manufacturing Optical Film>
The method for producing an optical film of the present invention includes a step of uniaxially stretching the above-mentioned film for producing an optical film. Hereinafter, a method for producing a polarizing film will be specifically described as an example of the method for producing an optical film.

Examples of methods for producing polarizing films include a dyeing process for dyeing a film for producing optical films (hereinafter also referred to as "PVA film"), a stretching process for uniaxial stretching, and, if necessary, a swelling process for swelling, a crosslinking process for crosslinking, a fixing process for fixing, a cleaning process for washing, a drying process for drying, a heat treatment process for heat treatment, etc. In this case, the order of each process is not particularly limited, but can be, for example, the swelling process, dyeing process, crosslinking process, stretching process, and fixing process. Also, one or more processes can be performed simultaneously, and each process can be performed twice or more times.

The swelling process can be carried out by immersing the PVA film in water. The temperature of the water during immersion is preferably 20°C or more and 55°C or less, more preferably 22°C or more and 50°C or less, and even more preferably 25°C or more and 45°C or less. The time for immersion in water is preferably, for example, 0.1 minutes or more and 5 minutes or less, and more preferably 0.5 minutes or more and 3 minutes or less. The water used for immersion in water is not limited to pure water, and may be an aqueous solution in which various components are dissolved, or a mixture of water and an aqueous medium.

The dyeing process can be carried out by contacting the PVA film with a dichroic dye. An iodine-based dye is generally used as the dichroic dye. The timing of dyeing may be any stage before, during, or after uniaxial stretching. A method of dyeing by immersing the PVA film in a dye bath containing iodine and potassium iodide (particularly an aqueous solution) is preferably used. The concentration of iodine in the dye bath is preferably 0.01% by mass to 0.5% by mass, and the concentration of potassium iodide is preferably 0.01% by mass to 10% by mass. The temperature of the dye bath is preferably 20°C to 50°C, particularly 25°C to 40°C. The preferred dyeing time is 0.2 minutes to 5 minutes.

By carrying out a crosslinking process to crosslink the modified PVA in the PVA film, it is possible to effectively prevent the modified PVA from dissolving into water during wet stretching at high temperatures. From this viewpoint, it is preferable to carry out the crosslinking process after the dyeing process and before the stretching process. The crosslinking process can be carried out by immersing the PVA film in an aqueous solution containing a crosslinking agent. As the crosslinking agent, one or more boron compounds such as boric acid and borax and other borates can be used. The concentration of the crosslinking agent in the aqueous solution containing the crosslinking agent is preferably 1% by mass or more and 15% by mass or less, more preferably 1.5% by mass or more and 7% by mass or less, and even more preferably 2% by mass or more and 6% by mass or less. By having the concentration of the crosslinking agent within the above range, sufficient stretchability can be maintained. The aqueous solution containing the crosslinking agent may contain potassium iodide, etc. The temperature of the aqueous solution containing the crosslinking agent is preferably 20°C or more and 60°C or less, particularly 25°C or more and 55°C or less. By setting the temperature within the above range, crosslinking can be carried out efficiently.

The stretching process for uniaxially stretching the PVA film may be performed by either a wet stretching method or a dry stretching method. In the case of the wet stretching method, it may be performed in an aqueous solution containing boric acid, or in the dyeing bath described above or in a fixing treatment bath described later. In the case of the dry stretching method, the stretching may be performed at room temperature, or may be performed while heating, or may be performed in air using the PVA film after absorbing water. Among these, the wet stretching method is preferred because it can be stretched with high uniformity in the width direction, and uniaxial stretching in an aqueous solution containing boric acid is more preferred. The concentration of boric acid in the aqueous solution of boric acid is preferably 0.5% by mass or more and 6.0% by mass or less, more preferably 1.0% by mass or more and 5.0% by mass or less, and particularly preferably 1.5% by mass or more and 4.0% by mass or less. In addition, the aqueous solution of boric acid may contain potassium iodide, and the concentration of potassium iodide is preferably 0.01% by mass or more and 10% by mass or less. The stretching temperature in the uniaxial stretching is preferably 30°C or more and 90°C or less, more preferably 40°C or more and 80°C or less, further preferably 50°C or more and 75°C or less, and particularly preferably 55°C or more and 68°C or less.

The stretching ratio in uniaxial stretching (total stretching ratio from unstretched PVA film) is preferably 5 times or more, more preferably 5.5 times or more, from the viewpoint of the polarizing performance of the resulting polarizing film. There is no particular upper limit to the stretching ratio, but the stretching ratio is preferably 8 times or less.

There is no particular restriction on the direction of uniaxial stretching when uniaxially stretching a long PVA film, and uniaxial stretching in the long direction or uniaxial stretching in the transverse direction can be used. Uniaxial stretching in the long direction is preferred because it results in a polarizing film with excellent polarization performance. Uniaxial stretching in the long direction can be performed by using a stretching device equipped with multiple rolls parallel to each other and changing the peripheral speed between each roll. On the other hand, uniaxial stretching in the transverse direction can be performed using a tenter-type stretching machine.

The maximum stretching stress in the stretching step is preferably 30 N/ ^mm2 or less. Here, the maximum stretching stress is the maximum tensile force applied to the PVA film divided by the cross-sectional area of the raw material PVA film. By reducing the maximum stretching stress, a polarized film with low shrinkage stress can be obtained. The maximum stretching stress is more preferably 25 N/ ^mm2 or less. In addition, the maximum stretching stress is usually 1 N/mm2 or ^more . The maximum stretching stress in the stretching step can be reduced by increasing the stretching temperature.

In the manufacture of polarizing films, a fixing process can be performed after the stretching process in order to strengthen the adsorption of the dichroic dye (iodine-based dye, etc.) to the PVA film. The fixing bath used for the fixing process can be an aqueous solution containing one or more boron compounds such as boric acid and borax. If necessary, an iodine compound or a metal compound may be added to the fixing bath. The concentration of the boron compound in the fixing bath is preferably 2% by mass or more and 15% by mass or less, particularly 3% by mass or more and 10% by mass or less. By setting the concentration of the boron compound within the above range, the adsorption of the dichroic dye can be made stronger. The temperature of the fixing bath is preferably 15°C or more and 60°C or less, particularly 25°C or more and 40°C or less.

The cleaning process is generally performed by immersing the film in distilled water, pure water, an aqueous solution, or the like. In this case, it is preferable to use an aqueous solution containing an iodide such as potassium iodide as an auxiliary agent from the viewpoint of improving the polarization performance, and the concentration of the iodide is preferably 0.5% by mass to 10% by mass. In addition, the temperature of the aqueous solution in the cleaning process is generally 5°C to 50°C, preferably 10°C to 45°C, and more preferably 15°C to 40°C. By setting the temperature of the aqueous solution within the above range, it is possible to further improve the polarization performance.

There are no particular limitations on the conditions for the drying process, but it is preferable to dry the PVA film at a temperature of 30°C or higher and 150°C or lower, and particularly 50°C or higher and 130°C or lower. Drying at a temperature within the above range makes it easier to obtain a polarizing film with excellent dimensional stability.

In addition, optical films other than polarizing films, such as retardation films, can also be manufactured by a method that includes a step of uniaxially stretching the film for optical film manufacturing of the present invention. As for the specific manufacturing method, a conventionally known method can be adopted, except that the film for optical film manufacturing of the present invention is used.

<Optical film>
The optical film of the present invention comprises a PVA having a silicon-containing group and an ethylene unit (modified PVA), wherein the silicon-containing group is a silanol group or a group that can be converted into a silanol group in the presence of water, the content of the silicon-containing group relative to all structural units in the modified PVA is 0.01 mol % or more and 1.0 mol % or less, and the content of the ethylene unit is 0.5 mol % or more and 10 mol % or less.

The optical film of the present invention may be an optical film obtained by the above-mentioned manufacturing method using the film for optical film production of the present invention. Details such as the specific structure and content of the modified PVA contained in the optical film of the present invention are the same as those of the modified PVA contained in the film for optical film production of the present invention. In addition, the optical film of the present invention may contain other components similar to those of the film for optical film production of the present invention.

The optical film of the present invention may be a polarizing film, a retardation film, a viewing angle improving film, a brightness improving film, etc., and is preferably a polarizing film. In this case, the polarizing film usually contains a dichroic dye, and the modified PVA may be crosslinked.

The optical film of the present invention is preferably a stretched film, more preferably a uniaxially stretched film. The optical film of the present invention may be a single-layer film or a multi-layer film, but is preferably a single-layer film. When it is such a film, the optical film of the present invention can be more suitably used as a polarizing film, etc.

The shrinkage stress of the optical film of the present invention is preferably 100 N/ ^mm2 or less. The small shrinkage stress results in excellent dimensional stability even when used at high temperatures. The shrinkage stress is more preferably 80 N/mm2 or ^less . The lower limit of the shrinkage stress of the optical film of the present invention may be, for example, 10 N/mm2, 20 N/ ^mm2 , or 40 N/ ^mm2 . Here, the shrinkage stress is a value obtained by dividing the tension when both ends ^of a polarizing film sample in the stretching direction are fixed and the film is maintained at 80°C for 4 hours by the cross-sectional area of the sample, and specifically refers to a value obtained by the method described in the Examples below.

When the optical film of the present invention is a polarizing film, it is preferable that the dichroic ratio (R) of the polarizing film is 100 or more. By including the above-mentioned modified PVA, a polarizing film having such a high dichroic ratio (R) can be produced with good productivity. The dichroic ratio (R) is more preferably 150 or more, even more preferably 190 or more, and particularly preferably 200 or more. The upper limit of this dichroic ratio (R) may be, for example, 350, or may be 300 or 250.

The method for calculating the dichroic ratio (R) of a polarizing film is as follows. First, the relationship between the transmittance (T') excluding surface reflection and the single transmittance (T) is shown by formula (a). In this case, the refractive index of the polarizing film is assumed to be 1.5, and the reflectance at the surface is assumed to be 4%. The relationship between the transmittance (T'), the degree of polarization (V), and the dichroic ratio (R) is shown by formula (b). Therefore, the single transmittance (T) and the degree of polarization (V) are measured, and then the dichroic ratio (R) of the polarizing film can be calculated by solving formulas (a) and (b) using these values.
T'=T/(1-0.04) ² ...(a)
R={-ln[T'(1-V)]}/{-ln[T'(1+V)]} ...(b)

A polarizing film is usually used as a polarizing plate by laminating an optically transparent and mechanically strong protective film on one or both sides of the polarizing film. Examples of protective films that can be used include cellulose triacetate (TAC) film, cycloolefin polymer (COP) film, cellulose acetate butyrate (CAB) film, acrylic film, and polyester film. Examples of adhesives used for lamination include PVA adhesives, urethane adhesives, and acrylate UV-curable adhesives. In other words, a polarizing plate has a polarizing film and a protective film laminated directly or via an adhesive layer on one or both sides of the polarizing film.

The polarizing plate can be used as a part of an LCD by, for example, coating it with an acrylic or other adhesive and then bonding it to a glass substrate. In addition, the polarizing plate may also be bonded with a retardation film, a viewing angle improving film, a brightness improving film, etc.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. The methods of measurement and evaluation are as follows.

[Polymerization degree of PVA (viscosity average polymerization degree)]
For each PVA synthesized in the following synthesis examples, the viscosity average degree of polymerization was measured in accordance with the description of JIS K6726-1994.

[Degree of saponification of PVA]
For each PVA synthesized in the following synthesis examples, the degree of saponification was measured according to the description of JIS K6726-1994.

[Content of silicon-containing groups and ethylene units in PVA]
For each PVA synthesized in the following synthesis examples, the content of silicon-containing groups and ethylene units was measured from the proton NMR of the vinyl ester polymer before saponification. When measuring the proton NMR of the vinyl ester polymer before saponification, the vinyl ester polymer was purified by reprecipitation with hexane-acetone to completely remove unreacted monomers from the polymer, and then dried at 90°C under reduced pressure for 2 days, and then dissolved in _CDCl3 solvent for analysis.

[Film swelling degree]
Each film (film for optical film production) obtained in the following Examples and Comparative Examples was cut to 1.5 g and immersed in 1000 g of distilled water at 30° C. for 30 minutes. After immersion for 30 minutes, the film was taken out, the water on the surface was absorbed with filter paper, and the mass (N) was measured. The film was then dried in a dryer at 105° C. for 16 hours, and the mass (M) after drying was measured. The swelling degree of the film was calculated from the obtained mass (N) and mass (M) according to the following formula.
Swelling degree (%) = 100 x N/M

[Dichroic ratio of polarizing film (optical performance)]
A rectangular sample of 4 cm in the length direction of the polarizing film was taken from the center of the width direction of each polarizing film obtained in the following Examples and Comparative Examples. The single transmittance (T) and polarization degree (V) of this sample were measured using an integrating sphere spectrophotometer ("V7100" manufactured by JASCO Corporation) in accordance with JIS Z8722 (measurement method for object color) after luminosity correction in the visible light region with a C light source and a 2° visual field.

The dichroic ratio (R) of the polarizing film was calculated by solving the following formulas (a) and (b) from the obtained single transmittance (T) and polarization degree (V). Here, the refractive index of the polarizing film was set to 1.5, and the reflectance at the surface was set to 4%. The dichroic ratio (R) of the polarizing film thus produced under the temperature conditions of the uniaxial stretching treatment bath in the Examples and Comparative Examples was set to R ₀ .
T'=T/(1-0.04) ² ...(a)
R={-ln[T'(1-V)]}/{-ln[T'(1+V)]} ...(b)

[Shrinkage stress of polarizing film]
The shrinkage stress of each polarizing film obtained in the following Examples and Comparative Examples was measured using Shimadzu Corporation's autograph "AG-X" with a thermostatic chamber and a video extensometer "TR ViewX120S". For the measurement, a polarizing film that had been conditioned at 20°C/20% RH for 18 hours was used. After the thermostatic chamber of the autograph "AG-X" was set to 20°C, the polarizing film (15 cm in the length direction, 1.5 cm in the width direction) was attached to a chuck (chuck interval 5 cm), and the temperature of the thermostatic chamber was raised to 80°C at the same time as the start of pulling. The stretching direction of the polarizing film was set to the length direction, and the film was pulled in this direction. The polarizing film was pulled at a speed of 1 mm/min, and the pulling was stopped when the tension reached 2 N, and the tension was measured in this state for up to 4 hours. At this time, since the distance between the chucks changes due to thermal expansion, a marking sticker was attached to the chuck and the measurement was performed using a video extensometer "TR ViewX120S" so that the distance between the chucks could be corrected by the amount of movement of the marking sticker attached to the chuck. The shrinkage force of the polarized film was determined by subtracting the initial tension of 2N from the tension measurement value after 4 hours, and the shrinkage stress (N/ ^mm2 ) was determined by dividing this shrinkage force by the cross-sectional area of the sample. The shrinkage stress (SF) of the polarized films produced in this manner under the temperature conditions of the uniaxial stretching treatment bath in the Examples and Comparative Examples was determined to be SF ₀ .

[Dichroic ratio and stretching temperature of polarizing film at shrinkage stress of 75 N/ ^mm2 ]
In the following examples and comparative examples, polarizing films with different temperatures of the uniaxial stretching bath were obtained in the same manner, except that the temperature of the uniaxial stretching bath was 2°C or 4°C lower, and the iodine concentration of the dyeing bath was changed so that the transmittance of the obtained polarizing film was 44.0%. The single transmittance (T) and polarization degree (V) of each obtained polarizing film were measured, and the dichroic ratio (R) was calculated by the above-mentioned method. Here, the dichroic ratio (R) of the polarizing film obtained under the condition of a uniaxial stretching bath temperature 2°C lower was R _-2 , and the dichroic ratio (R) of the polarizing film obtained under the condition of a uniaxial stretching bath temperature 4°C lower was R _-4 . In addition, the shrinkage stress of each polarizing film was measured by the above-mentioned method. Here, the shrinkage force (SF) of the polarizing film obtained under the condition of a uniaxial stretching bath temperature 2°C lower was SF _-2 , and the shrinkage force (SF) of the polarizing film obtained under the condition of a uniaxial stretching bath temperature 4°C lower was SF _-4 .

From the dichroic ratio (R ₀ , R _-2 , R _-4 ) and shrinkage stress (SF ₀ , SF _-2 , SF _-4 ) values of the three types of polarizing films thus obtained with different uniaxial stretching bath temperatures, the relationship between the dichroic ratio and the shrinkage stress was plotted and linear approximation was performed to calculate the dichroic ratio at a shrinkage stress of 75 N/mm ^2. In addition, the relationship between the shrinkage stress and the temperature of the uniaxial stretching bath was plotted and linear approximation was performed to calculate the stretching temperature at a shrinkage stress of 75 N/mm ² .

[Synthesis Example 1] Synthesis of PVA-1 3000 g of vinyl acetate and 3.6 g of vinyltrimethoxysilane as a monomer having a silicon-containing group were charged into a 5 L pressurized reaction vessel equipped with a stirrer, a nitrogen inlet, an ethylene inlet, an initiator addition port, and a delay solution addition port, and the temperature was raised to 60°C, and the system was replaced with nitrogen by nitrogen bubbling for 30 minutes. Then, ethylene was introduced so that the reaction vessel pressure was 0.26 MPa. After adjusting the temperature inside the polymerization vessel to 60°C, 1.0 g of 2,2'-azobisisobutyronitrile (AIBN) was injected to start polymerization. During polymerization, ethylene was introduced to maintain the reaction vessel pressure at 0.26 MPa and the polymerization temperature at 60°C while injecting a 2 mass% methanol solution of vinyltrimethoxysilane, and the polymerization was stopped by cooling when the polymerization rate reached 20% after 3 hours. The amount of the methanol solution of vinyltrimethoxysilane added by the end of polymerization was 84 ml. Next, unreacted vinyl acetate was removed under reduced pressure to obtain a methanol solution of modified polyvinyl acetate (hereinafter sometimes abbreviated as modified PVAc). The concentration of the obtained methanol solution of modified PVAc was adjusted to 23.5% by mass, and NaOH methanol solution (10% by mass concentration) was added so that the alkali molar ratio (moles of NaOH/moles of vinyl ester units in modified PVAc) became 0.04, and saponification was performed. The obtained polyvinyl alcohol was washed with methanol. The degree of polymerization (viscosity average degree of polymerization) of PVA-1 obtained by the above operations was 3,300, the degree of saponification was 99.9 mol%, the content of silicon-containing groups was 0.2 mol%, and the content of ethylene units was 2.5 mol%.

Synthesis Examples 2 to 9: Synthesis of PVA-2 to PVA-9 PVA-2 to PVA-9 having the viscosity average degree of polymerization, degree of saponification, silicon-containing group content, and ethylene unit content shown in Table 1 were obtained in the same manner as in Synthesis Example 1, except that the ratio of the monomers used, the polymerization conditions, and the saponification conditions were appropriately adjusted.

[Example 1]
An aqueous solution containing 100 parts by mass of PVA-1, 10 parts by mass of glycerin as a plasticizer, and 0.1 parts by mass of sodium polyoxyethylene lauryl ether sulfate as a surfactant, and having a PVA content of 9.5% by mass, was prepared as a film-forming stock solution. This film-forming stock solution was dried on a metal roll at 80° C., and the resulting film was heat-treated in a hot air dryer at a temperature of 105° C. for 10 minutes to adjust the swelling degree to 200%, thereby producing a PVA film (film for producing optical films) with an average thickness of 30 μm.

A sample of 5 cm wide x 9 cm long was cut from the center of the width direction of the obtained PVA film so that a range of 5 cm wide x 5 cm long could be uniaxially stretched. This sample was immersed in pure water at 30 ° C for 60 seconds and uniaxially stretched 2.0 times in the length direction for swelling treatment. Next, it was immersed in an aqueous solution containing 0.05% by mass of iodine and 1.15% by mass of potassium iodide (dyeing treatment bath: temperature 32 ° C) for 120 seconds and uniaxially stretched 1.2 times (total 2.4 times) in the length direction to adsorb iodine. Next, it was immersed in an aqueous solution containing 2.6% by mass of boric acid (boric acid crosslinking treatment bath: temperature 32 ° C) for 120 seconds and uniaxially stretched 1.25 times (total 3.0 times) in the length direction. The film was then immersed in an aqueous solution (uniaxial stretching bath) containing 2.8% by mass of boric acid and 5% by mass of potassium iodide at 69° C., and uniaxially stretched to a total of 6.0 times in the length direction. The film was then washed by immersing for 5 seconds in an aqueous solution (washing bath) containing 1.5% by mass of boric acid and 3.5% by mass of potassium iodide at a temperature of 22° C. Finally, the film was dried at 80° C. for 4 minutes to obtain a polarizing film.

The dichroic ratio of the obtained polarizing film at a shrinkage stress of 75 N/ ^mm2 was determined by the method described above. The stretching temperature of the polarizing film at a shrinkage stress of 75 N/ ^mm2 was also determined by the method described above. The results are shown in Table 1 and FIG.

[Examples 2 to 3 and Comparative Examples 1 to 5]
Using the PVAs (PVA-2 to PVA-8) shown in Table 1, PVA films were produced and evaluated in the same manner as in Example 1, except that the PVA content in the film-forming solution and the heat treatment temperature were adjusted so that the average thickness of the PVA film would be 30 μm and the swelling degree would be 200%. Using the obtained PVA films, polarized films were produced and evaluated in the same manner as in Example ¹ , except that the temperature of the uniaxial stretching treatment bath and the iodine concentration of the dyeing treatment bath were changed so that the shrinkage stress of the obtained polarized film would be around 75 N/mm2 and the single-piece transmittance would be 44.0%. The results are shown in Table 1 and Figures 1 and 2.

[Comparative Example 6]
An attempt was made to prepare a film-forming solution using PVA-9 as the PVA, but the PVA did not dissolve, and a film-forming solution could not be prepared. As a result, a PVA film and a polarizing film could not be produced, and the dichroic ratio and stretching temperature of the polarizing film at a shrinkage stress of 75 N/ ^mm2 could not be measured.

As shown in Table 1 and Figure 1, when an optical film having a shrinkage stress of 75 N/ ^mm2 is produced using the films for optical film production (PVA films) of Examples 1 to 3, an optical film having a dichroic ratio of 200 or more can be obtained at a stretching temperature of 68°C or less. That is, when an optical film having the same shrinkage stress is produced using the films for optical film production of Examples 1 to 3, the stretching temperature is prevented from increasing, and an optical film having excellent optical performance can be obtained. In other words, the films for optical film production of Examples 1 to 3 can prevent an increase in shrinkage stress and an increase in stretching temperature while improving the optical performance of the obtained optical film.

As shown in Fig. 2, when comparing films for optical film production (PVA films) using PVA with the same polymerization degree, the following can be seen. Compared to Comparative Example 1 using unmodified PVA with a polymerization degree of 2400, Comparative Examples 4 and 5 using modified PVA with a polymerization degree of 2400 and a silicon-containing group content of 0.2 mol% or 0.4 mol% have a significantly higher dichroic ratio (optical performance) at a shrinkage stress of 75 N/ ^mm2 , but the stretching temperature is also significantly higher. That is, compared to the case where unmodified PVA is used, it can be seen that the PVA film using PVA having only silicon-containing groups can significantly improve the optical performance of the obtained optical film, but cannot suppress the increase in the stretching temperature. On the other hand, in Example 3, which uses a modified PVA with a degree of polymerization of 2400, a silicon-containing group content of 0.2 mol%, and an ethylene unit content of 4.5 mol%, the dichroic ratio (optical performance) at a shrinkage stress of 75 N/ ^mm2 is higher than that of Comparative Example 4, while the stretching temperature is almost the same and the stretching temperature can be suppressed from increasing. That is, compared to the case where a PVA having only a silicon-containing group is used, when an optical film with the same shrinkage stress is produced, the stretching temperature is suppressed from increasing and an optical film with excellent optical performance can be obtained by using a film for optical film production (PVA film) using a PVA having a silicon-containing group and an ethylene unit, and the stretching temperature is suppressed from increasing. In other words, the optical performance of the obtained optical film can be improved while suppressing an increase in shrinkage stress and an increase in the stretching temperature.

The film for optical film manufacturing of the present invention can be suitably used as a material for polarizing films, which are constituent materials of LCDs.

Claims (9)

The method includes a step of uniaxially stretching a film for producing a polarizing film,
The film for producing a polarizing film is
It comprises a polyvinyl alcohol having a silicon-containing group and an ethylene unit,
the silicon-containing group is a silanol group or a group that can be converted to a silanol group in the presence of water;
the content of the silicon-containing group relative to all structural units in the polyvinyl alcohol is from 0.01 mol % to 1.0 mol %, and the content of the ethylene unit is from 0.5 mol % to 10 mol %. The method for producing a polarizing film according to claim 1 , wherein the polyvinyl alcohol has a viscosity average degree of polymerization of 1,000 or more and 6,000 or less and a degree of saponification of 98.7 mol % or more. The method for producing a polarizing film according to claim 2 , wherein the product of the viscosity average degree of polymerization and the content of the silicon-containing group is 100 mol % or more and 2,000 mol % or less. The method for producing a polarizing film according to any one of claims 1 to 3, wherein the film for producing a polarizing film has an average thickness of 1 µm or more and 75 µm or less. The method for producing a polarizing film according to any one of claims 1 to 4, wherein the film for producing a polarizing film has a degree of swelling of 140% or more and 400% or less. The method for producing a polarizing film according to any one of claims 1 to 5 , wherein the film for producing a polarizing film is a non-stretched film. It comprises a polyvinyl alcohol having a silicon-containing group and an ethylene unit,
the silicon-containing group is a silanol group or a group that can be converted to a silanol group in the presence of water;
the content of the silicon-containing group relative to all structural units in the polyvinyl alcohol is from 0.01 mol % to 1.0 mol %, and the content of the ethylene unit is from 0.5 mol % to 10 mol %. The polarizing film according to claim 7 , having a shrinkage stress of 100 N/m ² or less. The polarizing film according to claim 7 or 8 , which is a single layer film.

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